1. Field of the Invention
The invention relates to the field of the production of mixtures comprising hydrogen and carbon monoxide by partial oxidation of light hydrocarbons, such as methane.
2. Description of the Related Art
Hydrogen is a gas which is widely used in chemistry. 500 million m3 of it are produced annually. Approximately 95% of this hydrogen is captive production used in refining, in petrochemicals, in the synthesis of methanol and for the production of ammonia.
Merchant hydrogen only represents a small fraction of this overall production and, for applications other than petrochemicals and fertilizers, it requires a greater or lesser state of purity.
In view of the increasing requirements for merchant hydrogen (a growth of approximately +10% per year is observed) and the future requirements anticipated in industry in general (chemicals, petro-chemicals, metallurgy, electronics, fine chemicals), in decentralized energy production and in clean and nonpolluting transportation means using fuel cells, and in view of the problems raised by the infrastructure for the distribution of hydrogen (transportation, storage, safety), it appears increasingly necessary to have available sources of hydrogen which can produce relatively low amounts of hydrogen in situ under conditions which are satisfactory from the viewpoint of profitability.
The bulk production of captive hydrogen is carried out by refiners and major chemical manufacturers by different methods.
The first of these methods is the steam reforming of hydrocarbons originating from oil or natural gas. This is a highly endothermic reaction carried out between 800 and 900° C. over a catalyst and under a pressure of approximately 15 atmospheres. The burners are situated outside the catalytic beds and the hydrocarbon/steam mixture is preheated by virtue of a heat exchanger which uses combustion gases. This process makes it possible to achieve H2/CO ratios of 3 to 5, according to the steam throughput.
The second of these methods is mixed reforming. This is a process which operates under autothermal conditions, where the thermal energy necessary for the steam reforming over a catalyst is contributed by the partial combustion of the methane to CO2. The H2/CO ratio of the gases produced here is lower: typically between 1.5 and 2.5.
The third of these methods is the partial oxidation of the same hydrocarbons. This process does not require a catalyst. Combustion is carried out at 1 300-1 400° C. without steam. This process is exothermic but produces less hydrogen than the preceding processes.
It should be noted that the steam reforming reactions mentioned above require catalysts, which are sensitive to the impurities present in hydrocarbons originating from oil or contained in natural gas. It is therefore necessary, prior to the reactions described, to carry out an exhaustive desulfurization of the hydrocarbons. In contrast, the partial oxidation also mentioned above makes it possible to use dirtier hydrocarbons but this process has the disadvantage of generating soot.
Once the H2/CO mixture has been produced according to one of the preceding methods, the reaction for the production of hydrogen by conversion of the CO in the presence of steam over a catalyst, according to the reaction:CO+H2O→CO2+H2  (1)can be promoted.
When it is desired above all to produce hydrogen, steam reforming seems to be the most advantageous process. In combination with the water gas reaction and with a process for the separation of the various gases produced, for example by Pressure Swing Adsorption (PSA), it makes it possible to obtain hydrogen of very good purity (comprising less than a few tens of ppm of impurity). The energy consumption, per kg of hydrogen produced, is approximately 40 kWh/kg, which represents an energy efficiency of approximately 83%.
Merchant hydrogen currently originates essentially from the following processes:
(a) the recovery of the hydrogen produced in chemical and refining dehydrogenation operations involving catalytic reforming or cracking;
(b) the diversion of a portion of the hydrogen produced by captive producers when it is in excess;
(c) the production of coke intended for the iron and steel industry;
(d) the electrolysis of sodium chloride solutions, where hydrogen is coproduced at the same time as chlorine.
Furthermore, small production units have been constructed which resort to the decomposition of hydrogen-rich molecules:                by thermal cracking of ammonia,        by catalytic reforming of methanol,        by electrolytic dissociation of water.        
The latter processes result in hydrogen with a high cost price because of the cost of the starting materials (in particular methanol) or of the energy consumption (5 kWh per m3 of hydrogen produced for electrolysis). The thermal cracking of ammonia is a process which does not consume very much energy but the use of ammonia makes it a solution which is not very advantageous from the environmental viewpoint.
An advantageous alternative is to use catalytic generators of H2/CO atmospheres for heat treatments (for example, for carrying out cementation treatments of metal components), to add a water gas reactor at the outlet to convert the CO to CO2 and hydrogen, and to separate the CO2, the nitrogen possibly present and the hydrogen by methods known for this purpose.
There exists an extensive literature describing this type of process. The basic idea is to carry out a partial oxidation of a mixture comprising a hydrocarbon and an oxidizing gas over a catalyst generally based on a noble metal, such as platinum, nickel, rhodium or palladium. In these processes, it is necessary to add additional heating by burners or by an electrical element in order to achieve a temperature level sufficient for the reforming reactions, which are endothermic, to be able to be initiated. Part of this literature describes processes having the aim of improving and of controlling the temperature gradient in such reactors.
Thus, document GB-A-1 598 825 discloses an endothermic gas generator for the production of an H2/CO mixture intended for heat treatment. It preferably uses pure oxygen in order to avoid the external heat supply necessary for good progression of the reaction (which the presence of nitrogen would render necessary).
Document EP-A-450 872 shows a reactor in which an endothermic reaction, such as the production of an H2/CO mixture by reforming of methane by water, is carried out. A burner is placed at the center of the reactor, which makes possible more efficient heating of the catalytic bed in which the reaction takes place.
The same principle is improved in document WO-A-90/03 218, by virtue of circulation of the combustion gases around the catalytic bed, which makes it possible to limit the radial and axial temperature gradients.
Document U.S. Pat. No. 4,869,730 shows a U-shaped reactor in which an endothermic gas reaction takes place which makes possible the formation of a CO/H2/N2 mixture. The reactor is heated externally by burners.
Provision has also been made to carry out the production of a hydrogen/CO mixture in an externally heated annular reactor. This reactor has a cylindrical general shape and comprises, in its central part, a cylinder sealed off at least at its lower end which can either be empty or filled with an inert material or equipped with a heating means, such as an electrical element or a burner. The annular space of the reactor is filled with a catalytic medium. In comparison with a cylindrical reactor filled over its entire cross section with the catalytic medium, the annular reactor exhibits the advantage of decreasing the thermal gradients within the catalytic medium. The thickness of the catalytic medium is reduced and the transmission of the heat contributed by the external heating means is thus carried out more favorably therein. The optional presence of a burner or of other heating means in the central part of the annular reactor is also favorable from this viewpoint.
Each of the processes described above has disadvantages which make it difficult to apply them to small- or medium-sized plants for typically producing less than 1 000 m3/h of hydrogen. The capital cost required by steam reforming processes is crippling for small plants. The electrolysis of water is a process which is simple to operate and which is more suited to in situ production of hydrogen; on the other hand, it is a major consumer of energy. The thermal cracking of ammonia and the catalytic reforming of methanol are advantageous in terms of energy; on the other hand, the cost of methanol and the safety and environmental problems related to the use of ammonia make these processes difficult to operate industrially. As regards the presence of burners which heat the endothermic region of a catalytic reactor, it results in plants which are not very compact and which are poorly suited to units which have to produce small amounts of hydrogen. It is certainly possible to use elements to provide this reheating but this proves to be costly in terms of electric current.
Another way of proceeding consists in producing a hydrogen/CO mixture by carrying out the partial oxidation of a hydrocarbon (methane or propane, for example) by CO2 according to the reactions:CH4+CO2→2CO+2H2  (2)or C3H8+CO2→6CO+2H2  (3)
An advantageous way of carrying out this reaction consists in using a cylindrical reactor, in the upper part of which is inserted a radiant oxygen burner, also cylindrical, with a diameter substantially smaller than that of the reactor, fed with hydrocarbon and with oxygen. This oxygen burner descends as far as into the lower part of the reactor, where it introduces the combustion gases H2O and CO2. This lower part of the reactor is filled with an inert packing (for example, based on alumina), in which emerge the said combustion gases originating from the oxygen burner, on the one hand, and in which arrive, on the other hand, the gases (hydrocarbon and CO2) necessary for reaction (2) or (3). It should be noted that the combustion reaction in the oxygen burner produces CO2 and steam, which are used in the reaction for the production of the hydrogen/CO mixture. All these gases are mixed inside the inert region and rise inside the reactor by passing through an annular region defined by the internal wall of the reactor and the external wall of the oxygen burner. It is in this annular region that reaction (2) or (3) takes place, as it is filled with a catalyst, which it is possible to maintain at a temperature of approximately 1 200° C. as the result of its contact with the wall of the oxygen burner, despite the highly endothermic natures of reactions (2) and (3). The steam provided by the oxygen burner is sufficient by itself alone to prevent the formation of soot and the blocking of the reactor; no external steam supply is necessary for this purpose. The hydrogen/CO mixture produced is discharged from the upper part of the reactor and can be used on site or stored for use ex situ. The CO content of the mixture produced by the generator can advantageously be adjusted by introducing more or less CO2 into the lower part of the reactor in addition to the CO2 produced by the oxygen burner. A small reactor is thus obtained.
In the partial oxidation reaction of methane (or of another light hydrocarbon, such as propane or butane) in order to obtain a hydrogen/CO mixture, a purely thermal cracking requires temperatures of the order of 1 700° C. to achieve very high degrees of conversion (greater than 99%) with sufficiently reduced reaction times, of the order of 0.1 s. As the mixture is outside the flammability limits and the adiabatic temperature is approximately 650° C., it is necessary to burn a portion of the fuel in order to achieve these temperature levels.
Nevertheless, a thermodynamic analysis shows that, if the heat recovery is correctly organized, it is possible to operate under autothermal conditions.
This principle was proposed in document U.S. Pat. No. 5,441,581 (reference may also be made, in this category, to document WO93/21350): the heat exchanger, which is of the gas-gas type, is incorporated in the process for the manufacture of a CO/hydrogen/nitrogen mixture from a hydrocarbon/air mixture. The heat exchanger makes it possible to preheat the hydrocarbon/air mixture. Nevertheless, in this plant, heat exchange only takes place by convection. To be effective, it requires large exchange surface areas and high flow rates, resulting in the production of relatively complex exchangers. It is also important to note that, as the cooling is carried out slowly, the CO decomposes to CO2 and carbon according to the Boudouard equilibrium, which results in the formation of soot.